1. Field of the Invention
This invention relates to an image processing method and apparatus for converting color separation signals into new color separation signals.
2. Description of the Related Art
Color CRT (cathode-ray tube) monitors, color hard copiers and the like are generally used as apparatuses for outputting visual color images.
A color CRT monitor forms a visual image according to an additive color mixture by modulating the intensities of emission levels of phosphors having three-color, i.e., R (red), G (green) and B (blue), coated on the surface of the monitor's tube. On the other hand, a color hard copier forms a color image on paper according to a subtractive color mixture using Y (yellow), M (magenta), C (cyan) and K (black) color materials.
The above-described two types of image display devices have different color reproduction capabilities in theory. As shown in FIG. 4, in general, the color CRT monitor has a wider color gamut than the color hard copier. Accordingly, an image displayed on the color CRT monitor cannot be faithfully reproduced by the color hard copier.
FIG. 4 illustrates color signals on an (a*, b*) plane by converting them into L*, a* and b* signals in a standard color space. In FIG. 4, reference numeral 301 represents a color gamut of a typical color CRT monitor, and reference numeral 302 represents a color gamut of a typical color printer.
When two colors represented by points A and B are present in an image, the image displayed on the color CRT monitor is reproduced in different colors. When providing a hard copy of the image, the colors at the points A and B are both reproduced as a color at a point C. Hence, the two colors cannot be discriminated from each other in the hard copy, and therefore information contained in the image is lost.
In order to solve such a problem, a method relating to gamut mapping is performed, in which when providing a hard copy of an input color image, color signals in the image are converted into signals so as to be within the color gamut of the obtained hard copy. That is, by performing conversion so that the points A and B are converted into points D and E shown in FIG. 4, the colors can be discriminated from each other on the hard copy.
In general, in order to cause colors reproduced on the hard copy to approach colors on the color CRT monitor as much as possible, conversion is performed based on hue elements of colors.
That is, conversion is performed so that hues of color signals in a uniform color space do not change after color conversion. More specifically, as shown in the second quadrant of FIG. 4, an input color signal is converted into a color signal present on a line obtained by connecting the origin and the input signal in a uniform color space. This is because the sense of incongruity in visual characteristics of man is small when a color displayed on the color CRT monitor and a color displayed on an output hard copy have the same hue, even though they have different color saturation values.
However, even if the two colors have the same hue in the uniform color space, the actually output color is not always seen to have the same hue by human eyes. For example, it is more preferable to shift a blue color (represented by a point P shown in FIG. 4) to a direction of green (as represented by a point R shown in FIG. 4) than preserving the same hue (as represented by a point Q shown in FIG. 4), because the obtained color is seen to be closer to the original color.
That is, there is a problem in that visual characteristics of humans do not coincide with theoretical values based on a uniform color space.